Recently, a near-field optical accessing system is a promising technology for accessing a large number of data. A near-field optical disc drive is one type of the near-field optical accessing system. Generally, the near-field optical disc drive has an optical head. During normal operation of the near-field optical disc drive at the near-field position, the gap distance between a lens (e.g. a solid immersion lens) of the optical head and a surface of an optical disc is very small (for example approximately 200 nm or less).
Therefore, it is critical to move the lens to the near-field working position without causing collision between the optical head and the surface of the optical disc. Generally, the action of moving the lens to the working position (or a target position) is called as a lens pull-in action.
FIG. 1 is a graph illustrating a gap error signal (GES) generated by a near-field optical disc drive during the lens is moved from a far-field range to a near-field range. That is, the GES is generated by moving the lens from a far position relative to the optical disc to the working position. In the far-field range, the GES is maintained at a constant level because the light reflected from the optical disc is not received by the optical head. Whereas, in the near-field range, as the gap distance between the lens and the optical disc decreases, the GES level decreases. Until the lens is in contact with the optical disc, the GES level is zero. In other words, the distance between the lens and the optical disc may be expressed by the GES level. According to the feature of the GES, many lens pull-in methods have been disclosed.
For example, US. Patent Publication No. 2009/0154309 discloses a near-field optical disc drive and a lens pull-in method thereof. FIGS. 2A and 2B are graphs respectively illustrating the GES and the driving voltage processed by the lens pull-in method disclosed in US. Patent Publication No. 2009/0154309. From the time spot t0 to the time spot t3, by controlling the driving voltage, the lens of the near-field optical disc drive is firstly moved toward the optical disc. According to the time spots t1 and t3 when the slope of the GES changes, a first gap error value Vges1, a second gap error value Vges2, a first voltage value Vd1 corresponding to the first gap error value Vges1 and a second voltage value Vd2 corresponding to the second gap error value Vges2 are determined. Then, the lens is driven to be away from the optical disc, and a target value Vtarget is set to be equal to the average of the first voltage value Vd1 and the second voltage value Vd2. At the time spot t4, by controlling the driving voltage, the lens of the near-field optical disc drive is moved toward the optical disc again. At the time spot t5, the lens is confirmed to enter the near-field range. At the time spot t6 when the driving voltage reaches the target value Vtarget, the controlling circuit of the driving voltage is switched to a gap servo system. The gap servo system is a close loop control system. When the gap servo system is performed, the lens is stably operated by the driving voltage at the target value.
That is, for performing the lens pull-in action, the lens should be adjusted from the far-field position to the near-field position in an open loop state, and the slope of the GES is monitored to judge whether the lens is located in the near-field position but not in contact with the optical disc. After the driving voltage reaches the target value Vtarget, the lens is moved to the target position corresponding to the target value Vtarget, and the controlling circuit is switched to the gap servo system. However, the use of the open loop control system and the target value Vtarget to implement the lens pull-in action to move the lens to the target position is time-consuming.
Moreover, US. Patent Publication No. 2009/0290465 discloses another near-field optical disc drive and a lens pull-in method thereof. FIG. 3 is a graph illustrating associated signals processed by the lens pull-in method disclosed in US. Patent Publication No. 2009/0290465. As shown in FIG. 3, from the time spot t0 to the time spot t3, the near-field optical disc drive is operated in an open loop control state. According to the driving voltage, the lens is gradually moved toward the surface of the optical disc and reaches a target distance. According to the GES, it is found that the lens is in the far-field range from the time spot t0 to the time spot t1. In this situation, the lens is controlled to be moved toward the surface of the optical disc at a first moving speed according to the driving voltage. At the time spot t1 when the GES decreases to a first GES level, it is found that the lens enters the near-field range. Meanwhile, the lens is separated from the optical disc by a first distance. In this situation, the lens is controlled to be moved toward the surface of the optical disc at a second moving speed according to the driving voltage. At the time spot t2 when the GES decreases to a second GES level, the lens is separated from the optical disc by a second distance. In this situation, the lens is controlled to be moved toward the surface of the optical disc at a third moving speed according to the driving voltage. At the time spot t3 when the GES reaches a target GES level, the distance between the lens and the surface of the optical disc is the target distance. At the same time, the near-field optical disc drive is switched to a close loop control state. In addition, an inverse driving voltage having a pulse duration T and a pulse amplitude A is generated in order to prevent the optical disc from colliding with the lens. After the time spot t3, the optical disc drive is operated in a close loop control state, so that the lens is separated from the surface of the optical disc by the target distance. However, it is time-consuming to implement the lens pull-in action to move the lens to the target position according to several driving voltages when the near-field optical disc drive is operated in the open loop control state.
Similarly, US. Patent Publication No. 2009/0016179 also discloses a method of implementing the lens pull-in action by the open loop control system. This lens pull-in method is similar to that of the above literatures, and is not redundantly described herein.
Generally, for driving the lens in the open loop control state, the optical disc needs to be stationary. If the lens pull-in action is performed in the open loop control state to move the lens toward the rotating optical disc, a tiny disturbance of the optical disc may result in damage of the optical disc and the lens.